1. Field of the Invention
The present invention relates to a device for driving a cold cathode fluorescent lamp (CCFL) used as a backlight of a liquid crystal display.
2. Description of the Related Art
Similar to a conventional hot-cathode fluorescent lamp (xe2x80x9cFLxe2x80x9d) used for office and home lighting, CCFLs are high-efficiency, long-life light sources. By comparison, incandescent lamps have efficiency in the range of 15 to 25 lumens per watt, while both FLs and CCFLs have efficiency in the range of 40 to 60 lumens per watt. Furthermore, the average life of an incandescent lamp is only about 1,000 hours. However, FLs and CCFLs, on average, last for 10,000 hours or more.
The main difference between a hot-cathode FL and a CCFL is that the CCFL omits filaments that are included in a FL. Due to their simpler mechanical construction and high efficiency, miniature CCFLs are generally used as a source of back lighting for Liquid Crystal Displays (xe2x80x9cLCDsxe2x80x9d). LCDs, whether color or monochrome, are widely used as displays in portable computers and televisions, and in instrument panels of airplanes and automobiles.
However, starting and operating a CCFL requires a high alternating current (xe2x80x9cacxe2x80x9d) voltage. Typical starting voltage is around 1,000 volts AC (xe2x80x9cVacxe2x80x9d), and typical operating voltage is about 600 Vac. To generate such a high ac voltage from a dc power source such as a rechargeable battery, portable computers and televisions, and instrument panels, include a dc-to-ac inverter having a step-up transformer.
In the push-pull configuration illustrated in FIG. 1, Lk1 and Lk2 are the leakage inductances of the transformer T, DS1 and Cs1 are the body diode and internal capacitance of switch S1, respectively, and Ds2 and Cs2 are the respective body diode and internal capacitance of switch S2. Winding N3 is coupled with windings N1 and N2. Inductor Lr, is a resonant inductor including a leakage inductance of transformer T. Inductor Lr and capacitor Cr form a resonant tank to provide a high frequency voltage to the load, Ro.
FIGS. 2a-2d illustrate typical switching waveforms associated with the circuit of FIG. 1. Referring first to FIG. 2a, at the point in time when switch S1 is turned off (t0) energy stored in the leakage inductance Lk1 is released to charge the capacitance Cs1 which causes an undesirable voltage spike across switch S1, as illustrated in FIG. 2c. Another problem associated with the circuit configuration of FIG. 1 is that the high voltage spike requires that switches S1 and S2 have high voltage breakdown voltage ratings.
At time t1, the gate signal (See FIG. 2b) of switch S2 is applied allowing switch S2 to be turned on at zero voltage (not shown). S2 carries the primary winding current.
As shown in FIG. 2d, a second voltage spike occurs at time t2 at switch S2, the point at which switch S2 is turned off. This voltage spike is the result of the release of energy from the leakage inductance Lk2.
Referring now to FIG. 3, one prior art solution for eliminating or minimizing the undesirable voltage spikes is through the use of passive snubber circuits (R-C-D) for switch S1 and (R-C-D) for switch S2, respectively. The passive snubber circuits are designed to absorb the leakage energy of the transformer (Lk1, Lk2). An undesirable consequence of using snubber circuits is that the converter circuit has a lower conversion efficiency by virtue of having to dissipate the undesirable leakage energies.
Another type of conventional ballast, illustrated in FIG. 4, employs a half-bridge inverter circuit configuration. The half-bridge switching circuit includes switches S1 and S2, resonant inductor Lr and resonant capacitor Cr. Inductor Lr could represent the leakage inductance or a separate inductance in the case where the leakage inductance is insignificant. Cr could represent a combination of the winding capacitance and shield capacitance of the lamp. Cd represents a DC blocking capacitor. The input voltage, Vin, is typically around 12 V. Until the CCFL or load (RL) is xe2x80x9cstruckxe2x80x9d or ignited, the lamp will not conduct a current with an applied terminal voltage that is less than the strike voltage, e.g., the terminal voltage can be as large as 1000 Volts. Once an electrical arc is struck inside the CCFL, the terminal voltage may fall to a run voltage that is approximately ⅓ the value of the strike voltage over a relatively wide range of input currents. To achieve voltages on the order of 1000 volts, a high voltage gain of the resonant inverter is required in addition to a high turns ratio of the isolation transformer. However, given that the peak excitation voltage Vx of the resonant tank is only one-half the input voltage, the resonant inverter voltage gain is restricted. Therefore, the only means of achieving a strike voltage on the order of 1000 volts is to require that the transformer have a very high turns ratio. This is problematic, however, in that a high turns ratio transformer is characteristically leaky and therefore not efficient.
Accordingly, it is desirable to provide an improved ballast which is more efficient in operation than a conventional ballast whether of the push-pull or half-bridge type while reducing or substantially eliminating spike voltages.
Accordingly, it is an object of the invention to provide an inverter circuit which eliminates or substantially reduces voltage spikes associated with switching elements in a push-pull switch configuration.
It is a further object of the invention to provide an inverter circuit which recovers leakage energy associated with an isolation transformer to improve circuit efficiency.
It is yet a further object of the invention to provide an inverter circuit which reduces the turns ratio of the isolation transformer to reduce power losses in the transformer to further improve circuit efficiency.
In accordance with an embodiment of the present invention, there is provided an inverter circuit and a method for efficiently converting a direct current (DC) signal into an alternating current (AC) signal for driving a load such as a cold cathode fluorescent lamp. The inverter circuit includes a resonant tank circuit having a resonant inductor and resonant capacitor and coupled via a transformer between a DC signal source and a common terminal of a half-bridge switch configuration. A voltage clamping capacitor is connected to a second and third terminal of the half-bridge switch configuration. A voltage difference between the capacitor voltage and the supply (i.e., input) voltage is applied to the terminals of the resonant tank. The voltage difference across the resonant tank is nominally twice the voltage of prior art configurations.
The inverter circuit according to the present invention includes a primary circuit having a DC voltage supply, a transformer coupling said primary and load circuits, a switching circuit comprising a first switch and a second switch for controlling a conduction state of said inverter circuit; a tank circuit having a resonant inductor and a resonant capacitor, the lamp load being coupled with the resonant capacitor; and a capacitor coupled to the first and second switches for maintaining a voltage across a primary winding of said transformer.
Accordingly, the required turns ratio of the transformer is reduced by half, as compared to prior art inverter circuits, thereby reducing the power loss in the transformer which improves circuit efficiency.
In accordance with another aspect of the present invention, the leakage energy stored in a leakage inductance associated with the transformer is recovered or captured by the clamping capacitor thereby preventing or substantially reducing the occurrence of voltage spikes across the switches which comprise the half-bridge switching configuration. As described above, in one prior art configuration, this leakage inductance, when released, charges a capacitance associated with the push-pull switches which causes voltage spikes across the switches. An additional advantage of capturing the leakage current is that the voltage ratings of the switches is significantly reduced.